1. Field of the Invention
This invention relates to an optical apparatus, for example, an optical apparatus in an exposure apparatus which effects focusing, alignment, etc. of a mask pattern and a projected substrate to a projection optical system.
2. Related Background Art
The focusing and alignment system in the prior art exposure apparatus, as disclosed, for example, in U.S. Pat. No. 4,395,117, is of a construction in which both of an illuminating system from which an illuminating light is output and a light-receiving system to which the reflected light or the detection light is input are present on the mask or reticle side which is one object side.
That is, it is of a construction in which the illuminating light beam incident from the reticle side enters a wafer which is the other object side through a reticle and a projection optical system and is reflected thereby and transmitted through the projection optical system and the reticle and again returns to the incidence side.
Therefore, it is necessary to divide the incident light and the reflected light or the detection light by the use of light dividing means such as a half-mirror. This leads to a considerable loss of quantity of light, which in turn leads to the inconvenience that sufficient alignment accuracy or focusing accuracy cannot be obtained.